I came across an article on Academia.edu that mentioned the following about Christian of Mainz, Archbishop in the service of Emperor Frederick Barbarossa:

“In the battle for Bologna it was recorded that, clad in full armour and on horseback, the bloodthirsty prelate [Christian of Mainz] killed nine men with a triple-headed bludgeon.”

Does anyone know what the primary source for this is, and what it says in Latin? Also, how should we interpret this weapon? Naturally, one tends to think of a flail, but my perception is that flails are very rare in this period and that tripled headed flails are primarily found in the 15th century and later.

Unfortunately, I do not recall which Academia article this came from. I think it was to do with clerics who played violent roles during the Middle Ages.

The article is here

https://deremilitari.org/wp-content/uploads/2013/10/arnold.pdf (see page 164)

The information appears to come from the Annales Stadenses (MGH, Scriptores, 16), 347.

My first thought was that the writer is referring to a mace with 3 prominent spikes or knobs. As always with translations you are never sure how accurate they are - you need the original text. If someone can produce the Latin original I could translate it for you.
Neil